1. Field of the Invention
The present invention relates to a silicon micro pressure sensor or transducer. More particularly, the invention relates to a monitoring and recording silicon active pressure sensor or transducer suitable for long term implantation in the body.
2. Description of Related Art
In order to monitor pressure within certain environments a pressure transducer or sensor is encapsulated in a hermetically sealed packaging or enclosure to protect its electronic circuitry against malfunction. EP Patent No. 1 184 351 discloses a method for brazing two glass components together to form a leak tight container for encapsulating electronic components such as a pressure transducer implanted in a human body. Many medical applications require implantable measurement devices, for example, implantable pressure transducers or sensors. Once implanted, the passive pressure transducer may be inductively coupled to a reading unit which energizes the transducer and allows the transmission between the transducer and the reading unit of data corresponding to the measured pressure.
One disadvantage associated with a passive pressure sensor activated by external RF energy only during communications with an external device is that the monitoring and recording of pressure data is dependent on communication with the external device and hence not continuous. An active pressure sensor including an internal battery is therefore desirable so that the pressure data may be monitored and recorded continuously or at least independent of communication with an external device. However, such an active implantable pressure sensor raises several design challenges. A hermetically sealed glass encapsulation of an active implantable pressure sensor is impractical due to the relatively large size requirements in order to accommodate the internal battery that would make the glass too fragile.
Another known approach disclosed in U.S. Pat. No. 6,543,291 is to encapsulate a pressure sensor in a hermetically sealed metallic enclosure resistant to certain chemical environments. Pressure outside the sealed enclosure is transmitted across its boundary by compression of a pressure transmitting medium, typically oil or mercury, that when displaced mechanically deflects a relatively thin membrane welded onto the metallic enclosure. Outside pressure deflects the metallic membrane displacing the medium within the metallic enclosure and a deflectable membrane associated with the pressure sensor element. An electrical signal produced by the electronic circuitry of the pressure sensor element is then converted to a pressure data signal by signal processing circuitry. The use of a fluid as a pressure transmitting medium may undesirably damp the measured pressure signal due to the difficulty in zeroing the compliance of the transmitting medium during manufacture of the sensor. For instance, during manufacture the welding of the metallic membrane to a titanium enclosure may cause the formation of unwanted air bubbles in the pressure transmitting medium that could damp the measured pressure signal. U.S. Pat. No. 6,450,038 discloses a mechanical transmitting mechanism rather than the fluid pressure transmitting medium. A silicon deflectable membrane of a silicon pressure sensor element would be too fragile to be in contact with a mechanical transmitting mechanism, such as the mechanism disclosed in U.S. Pat. No. 6,450,038.
It is therefore desirable to develop an improved implantable active silicon pressure sensor that is hermetically sealed to prevent leakage when used in a fluid environment without the drawbacks of the conventional apparatus heretofore used.